A manufacturer knows that the number of items produced per hour by machine A and by machine B are normally distributed with a standard deviation of 4.6 items for machine A and a standard deviation of 6.8 items for machine B. The mean hourly amount produced by machine A for a random sample of 50 hours was 230 units; the mean hourly amount produced by machine B for a random sample of 45 hours was 215 units. Find a 90% confidence interval for the difference in mean parts produced per hour by these two machines.